The present invention relates, in general, to transfer molds and, more particularly, to vacuum assisted transfer molds.
One use of transfer molds is to encapsulate electronic devices in plastic. In particular, semiconductor devices (die) mounted on some form of lead frame are encapsulated in plastic. This encapsulation is accomplished by placing the leadframe, with the die attached, on a bottom mold plate. A top mold plate is then lowered onto the bottom mold plate. A cavity is formed by the mold plates about the die. A clamping pressure is exerted on the mold plates while a ramp tip is inserted in an opening, or pot. A series of runners couple the cavities of the mold to the pot area. Liquid plastic is then forced through the pot and runners, into the cavities.
A problem which arises in this process is from air being trapped within the cavity area. This can create voids in the plastic which, in the worst case, will not cover the semiconductor die.
This problem is compounded in some devices which have heat sinks incorporated within the plastic. The thickness of plastic about the heat sink is required to be thin, on the order of 0.010 to 0.015 inches, to minimize the degradation in heat transfer. Because of the location and the thinness of the plastic required, this is greatly effected by voids. A void can allow electrical contact or arcing between the heat sink and the surface to which it is mounted.
Accordingly, it is an object of the present invention to provide a mold system that overcomes the above deficiencies.
A further object of the present invention is to provide a mold system which utilizes a vacuum.
Another object of the present invention is to provide a mold system with a self cleaning vent pin.
Still another object of the present invention is to provide a mold system for encapsulating semiconductor devices.
The above and other objects and advantages of the present invention are provided by the vent pin and mold system described herein.